i think the big image ad draws a bit more attention to the text ads
nice tidbit, reasonable and never really tried out.
apart from the content, i could imagine a high traffic and high page view (= ad view) website would better fit with display ads? mobile traffic is another thing, not sure about that at the moment. but strange thing also, that no display ads were paid per view - only cpc ads in my account.
i think you really have to give attention on how display ads actually perform compared to text ads. apart from the design issues, for me at least.